Abstract
The Asama volcano erupted on 1 September 2004, followed by several more eruptions during the period from September to December 2004. In the proximal area, particularly on the western side of the crater, the distribution and details of ejecta from the September 1 eruption have not been well studied. The purpose of this study is to investigate the ejecta from this area, and to obtain information concerning the juvenile magma of one eruption. Sasaki and Mukoyama (2006) presented a distribution map of the impact craters of the September 1 eruption, using IKONOS high-resolution satellite imagery; this imagery is partially effective for determining the distribution of ejecta from the eruption. However, ejecta in deep forests are not detectable by IKONOS imagery, and the rock types of the ejecta cannot be ascertained from imagery alone. Therefore, we collected field observations on the distribution of ejecta, to evaluate the effectiveness of IKONOS imagery data. After examining 130 impact craters identified by Sasaki and Mukoyama (2006), we determined that more than 100 of the impact craters (of various sizes) had been formed by impacts of massive and/or altered accessory blocks. At the other 30 spots, we discovered three juvenile breadcrust bombs. Moreover, we discovered 24 bombs at sites not detected by the IKONOS imagery. These results indicate that a combination of satellite imagery and field surveys is required to obtain precise distributions and details of volcanic ejecta. The water contents in the glass rinds of the 22 breadcrust bombs were from 0.49 to 0.86 wt.%. Calculations of magma depth on the basis of these water contents indicate that the magma column was at least 180 m high.
